Validation for 588

[jenniferward]

In the templates for printed music, the field 588 is open and yellow. Catalogers in a hurry sometimes start putting notes there (example: https://muscat-test.rism.info/admin/sources/1001131300), and then forget a proper 588 because the validation see text in 588. Or they just enter their shelfmark, which can become hard to interpret if eventually many more holdings are added. The proper form in 588 is siglum and shelfmark.

Can there be a validation for 588? At the very least there should be a - to signal a siglum. That might be enough to make catalogers look again.

[xhero]

For the moment it just checks for "-", in the next release we could make it more clever